1. Field of the Invention
This invention relates to a method of and a device for detecting a fault when making welded joints in thermoplastic pipe using so-called electro-fusion fittings.
An electro-fusion fitting generally comprises a body of thermoplastic material having an electrical resistance heating element disposed adjacent a surface of the body and to which an electric current may be supplied to fuse the material of the body to that of a pipe. The fittings are frequently used in the production of joints between lengths of thermoplastic pipe, such as polyethylene pipe which is used in the gas industry. A fitting for joining two pipes end-to-end usually comprises a sleeve or muff incorporating a coil of resistance heating wire adjacent its inner surface. The ends of the pipes to be connected are pushed into the sleeve, the heating coil is connected to a suitable power supply and an electric current is supplied to the coil for a controlled period to soften and fuse the material of the fitting and pipes so that they become firmly welded together.
Pipe joints obtained by use of electro-fusion fittings are generally satisfactory, but the weld strength achieved depends upon the correct level of electric current being supplied for the correct amount of time, and these factors differ for fittings of different sizes and types.
2. Prior Art
In EP-A No. 0076043 there is described a control system which sets automatically the correct time for which current is supplied to a fitting, whereby the manufacture of satisfactory joints is simplified. The standard of joint obtained is also dependent upon the pipes and sleeve being correctly assembled together before delivering the current to the heating coil. Thus, if the pipe ends are: (i) too far apart; (ii) not centred within the sleeve; (iii) not square to each other; or (iv) misaligned, a faulty joint can result and may not be detectable by visual inspection of the completed joint. When the pipes and sleeve are correctly assembled the flow of molten material is small during the welding process, but if they are not properly assembled the flow can be fairly substantial leading to adjacent turns of the heating coil becoming pushed together and shorting. As a consequent over-heating may occur and in extreme cases the current may rise to such a level that the wire may melt and break, possibly causing a spark which can be highly dangerous if gas is present. The present invention is based on the realisation that an unexpected rise in the electric current flowing in the heating coil is indicative of improper conditions during the welding process.